Archaeology long wrestled with a great problem: there was no objective way to say how old a bone, a piece of charcoal, or a wooden tool was. Dates were estimates based on stacked layers of soil and on comparison. In 1949 the American chemist Willard Libby (1908–1980) brought a clock to this problem.

The principle the method rests on is an elegant natural fact. A small but constant fraction of the carbon in the atmosphere is a radioactive kind formed by cosmic rays: carbon-14. All living things, as long as they live, take this carbon in by breathing and feeding; so every living organism has the same proportion of carbon-14 as the atmosphere. But when the organism dies the exchange stops, and the carbon-14 within it begins to decay radioactively at a known and unchanging rate (half of it in about 5,730 years). By measuring how much carbon-14 remains in a sample, you can calculate how long ago death occurred — a kind of counting-down hourglass.

Libby tested his method on samples of known age (for example wood from dated Egyptian tombs); the results held. Radiocarbon dating could date organic material back to about 50,000 years — exactly the critical span of human history and prehistory.

The impact was revolutionary. From Göbeklitepe to Ice Age cave paintings, from Egyptian mummies to the Shroud of Turin, countless finds could now be dated objectively; archaeology moved from estimate to measurement. Libby received the Nobel Prize in Chemistry in 1960. In later years the method was refined further, calibrated with tree rings. An unexpected gift of radioactivity — of Curie's discovery — became a clock that could measure the past.